The Collaborative International Dictionary of English v.0.48:

Insignificant \In`sig*nif"i*cant\, a.
   1. Not significant; void of signification, sense, or import;
      meaningless; as, insignificant words.
      [1913 Webster]

   2. Having no weight or effect; answering no purpose;
      unimportant; valueless; futile.
      [1913 Webster]

            Laws must be insignificant without the sanction of
            rewards and punishments.              --Bp. Wilkins.
      [1913 Webster]

   3. Without weight of character or social standing; mean;
      contemptible; as, an insignificant person.

   Syn: Unimportant; immaterial; inconsiderable; small;
        inferior; trivial; mean; contemptible.
        [1913 Webster]

WordNet (r) 3.0 (2006):

insignificant
    adj 1: not worthy of notice [syn: insignificant,
           undistinguished]
    2: signifying nothing; "insignificant sounds"
    3: of little importance or influence or power; of minor status;
       "a minor, insignificant bureaucrat"; "peanut politicians"
       [syn: insignificant, peanut]
    4: devoid of importance, meaning, or force [syn:
       insignificant, unimportant] [ant: important,
       significant]

Moby Thesaurus II by Grady Ward, 1.0:

74 Moby Thesaurus words for "insignificant":
   aimless, back-burner, casual, cursory, depthless, designless,
   dinky, dispensable, empty, few, footling, garbled, immaterial,
   importless, inane, inappreciable, inconsequential, inconsiderable,
   inessential, inferior, insubstantial, irrelevant, lesser, light,
   little, low, meager, meaningless, miniature, minor, minute,
   negligible, niggling, no great shakes, nonconnotative,
   nondenotative, nonessential, not vital, nugatory, null, paltry,
   petit, petty, phatic, picayune, picayunish, piddling, pointless,
   puny, purportless, purposeless, scrambled, secondary, senseless,
   shallow, shoestring, short, skin-deep, small, small-beer,
   small-fry, small-time, superficial, technical, tiny, trifling,
   trivial, unessential, unimportant, unimpressive, unmeaning,
   unnoteworthy, unsignificant, unsubstantial